1. Field of the Invention
The invention relates generally to measuring instruments and more particularly to instruments for measuring the rotation rate of a fiber gyro.
2. Description of the Prior Art
In a fiber gyro, a light beam is split into two component beams which travel through an optical fiber in opposite directions. The phase difference between the two component beams emerging from the optical fiber is a measure of the rotation rate of the optical fiber. An alternating voltage signal derived from the two component beams is employed to evaluate the phase difference .phi..sub.s between the component beams. An instrument of the type described herein is discussed in German patents DE-OS Nos. 31 36 688 and 31 23 163. Both patents describe an optical system having a modulated light beam, Patent DE-OS 31 36 688 describes a system, in which at least one of the two light beams to be coupled in a coiled optical waveguide is frequency-modulated. The second patent DE-OS 31 23 163 discloses a device in which at least one light beam is phase-modulated.
In integrated optical devices, phase modulation or frequency modulation is frequently utilized. The material commonly used for such integrated optical devices is LiNbO3, which has a higher refractive index (n approximately equals 2.2) than the material utilized in a coiled optical waveguide (n approximately equals 1.47) employed. A problem results in that reflections of the light beam occur at a junction where the light is coupled into a phase modulator from an optical fiber carrying the light beam, that is, reflections at the transition from the optical fiber to the phase modulator. The reflections of the light beam produce signal components which interfere with the evaluation of the modulated output signal. This is particularly the case if the reflected light beams are modulated at the same frequency as the component beams to be evaluated. Such interference is usually not troublesome if the light used has a short coherent wavelength. However, if the light used has a long coherent wavelength, the interference is greater but may be advantageous in certain situations such as when higher frequency stability is required. Notwithstanding these advantages, the interference in the modulated output signal resulting from the reflected light beam signal components continue to exist.